The most common electrodepositing baths for nickel electroplating are Watt's baths containing nickel sulfate, nickel chloride and usually boric acid; chloride baths containing nickel chloride and boric acid, and sulfamate baths containing nickel sulfamate, nickel chloride and usually boric acid. The latter baths are used for the more complicated platings and are difficult and comparatively expensive in use.
Corresponding platings of cobalt may be formed in similar baths containing cobalt sulfate and cobalt chloride instead of the corresponding nickel salts. By adding other metal salts platings of nickel or cobalt alloys are obtained.
It is known to employ a pulsating current, confer for instance W. Kleinekathofer et al, Metalloberfl. 9 (1982), page 411-420, where pulse plating is used by alternating between equal periods of a direct current with a current density of 1 to 20 A/dm.sup.2 and non-current periods, the pulse frequency being from 100 to 500 Hz. By employing a pulsating current and as result of the individual current impulses, an increased formation of crystal nucleuses is obtained, thus rendering a more fine-grained and hard plating.
It is furthermore known to employ pulse plating with periodic reverse pulse, i.e. alternating between a cathodic and anodic current. In the cathodic current cycle, the desired plating formation is obtained by metal deposition, while a portion of the deposited nickel is removed by dissolution in the anodic current cycle, any nodules in the plating thus being smoothed. In order to ensure that the, result is a build-up and not a dissolution of the plating, it is appreciated that the anodic load is to be less than the cathodic load. This method is e.g. described by Sun et al., Metal Finishing, May, 1979, page 33-38, whereby the highest degree of hardness in the plating is obtained at a ratio between the cathodic and the anodic current density of 1:1 with cathodic cycles T.sub.K of 60 msec. alternating with anodic cycles T.sub.A of 20 msec.
U.S. Pat. No. 2,470,775 (Jernstedt et al.) discloses a process for electroplating nickel, cobalt and alloys thereof in an electrodepositing bath containing chlorides and sulfates of the metals. The plating is effected by means of reversed pulse resulting in an improved appearance (smoothness and maximum brightness) as well as in an expedited deposition. An anodic current density is employed of substantially the same range as the cathodic current density. Various additives are mentioned in the U.S. patent, including naphthalene-1,5-disulfonic acid. These additives are referred to as advantageous components, however no directions are rendered in connection with these additives or elsewhere in the patent as to how the mechanical internal stresses are reduced in the platings resulting from electroplating.
EP patent No. 0.079.642 (Veco Beheer B.V.) relates to pulse plating with nickel in an electrolytic bath of the Watt's bath type comprising butynediol or ethylene cyanohydrin as brightener. The deposition is preferably performed at a pulsating current without anodic cycles, but it is stated that anodic cycles, i.e. reverse pulse, can also be employed with the same result. It is, however, not possible to use long anodic pulses in a pure Watt's bath without passivating the nickel layer, whereby any further deposition is prevented. Moreover, said patent discloses that the frequencies used are in a range from 100 to 10,000 Hz.
None of the above mentioned publications relate to internal stresses in platings. U.S. Pat. No. 3,437,568 relates to a method for measuring the internal stresses in electroformed parts, but does not advise how to reduce the internal stresses and does not relate to pulse plating, additives or special nickel baths.
DE published specification No. 2.218.967 discloses a bath for electrodeposition of nickel, to which bath a comparatively large amount of sulfonated naphthalene is added, such as from 0.1 mole/l to saturation so as to reduce the internal stresses in the platings applied by electroplating and with a direct current of e.g. 30 or 60 mA/cm.sup.2 corresponding to 3 to 6 A/dm.sup.2. According to the publication, the internal stresses are only reduced from the undesired tensile stress range to the compressive stress range from 0 to 26,000 psi (approx. 179 MPa) by employing this bath.
Usually, the use of said additive only results in a reduction in the stresses in the range from approx. 300 MPa tensile stress to 100 MPa compressive stress and the stress curve is merely moved downward, but is still a function of the current density, which is a normal condition for any type of nickel bath with or without additives.
The use of the large amount of additive is, however, also encumbered with several drawbacks, since the additive is expensive, has detrimental effects on the environment and may cause damage to the bath.
Thus, an electroplating method, wherein the internal stresses are independent of the current density, cannot be deduced from the teachings of DE 2.218.967. When electroplating members of a simple geometric shape, often comparatively modest variations in the current density occur over different areas of the surface of the members. However, this is not possible when dealing with more complicated geometric shapes, wherein the method according to DE 2.218.967 cannot be employed in practise.
Internal mechanical stress is a problem in all nickel and cobalt depositions, even though the process can be controlled satisfactorily in some instances (by means of expensive electrolytes (sulfamate bath), temperature control, concentration, etc.) when dealing with simple geometric shapes. The prior art methods are, however, completely inapplicable for the manufacture of tools for injection moulding, micro mechanical components or similar complicated geometric shapes.
Consequently, it is desirable to provide a method, whereby nickel, cobalt, nickel or cobalt alloys can be deposited with substantially reduced or completely without internal stresses--even in complicated geometric shapes. It is also desirable that this result is obtained whichever current density is used for the deposition.